Cardiac rhythm management (CRM) devices can help assist heart function, such as by providing pacing electrostimulations to evoke responsive heart contractions, providing cardiac resynchronization therapy (CRT) electrostimulations to coordinate the spatial nature of a heart contraction of one or more heart chambers, providing antitachyarrhythmia pacing, cardioversion, or defibrillation shocks to interrupt a tachyarrhythmia, or providing neurostimulation to influence sympathetic or parasympathetic nervous system response.
Diagnosing the physiological condition of a patient can involve determining whether fluid accumulation has occurred. For example, congestive heart failure (CHF) patients can have poor cardiac output (CO) from the heart. This can lead to fluid buildup in the lungs (e.g., pulmonary edema) or in the limbs (e.g., peripheral edema). Fluid accumulation status can be monitored by monitoring tissue impedance. Tissue impedance monitoring can involve injecting a test current into the tissue, and monitoring a resulting voltage. The resulting voltage can provide an indication of tissue impedance. The tissue impedance can provide an indication of how wet the tissue is. As the tissue becomes wetter, its impedance decreases.
Wang U.S. Pat. No. 7,149,573 discloses an example of tissue impedance monitoring, including determining contributing physiological impedance factors, such as lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume and lung volume. (See Wang '573 at Abstract.) Wang's impedance determination apparently relies upon parallel vectors—Wang's test current is injected between electrodes defining a test current vector therebetween, and the resulting voltage is measured between electrodes defining a response voltage vector therebetween, and the response voltage vector is substantially parallel in direction to the test current vector. (See, e.g., Wang '573 at col. 9, lines 30-60.)